Egg formation and structure

Physiological studies have centred chiefly on the pseudophyllidean and cyclophyllidean egg (Fig. 7.1). The formation of the capsule or egg shell (p. 171) has been studied extensively and these processes are dealt with in detail below. The field has been the subject of a number of reviews (138,170,451-453, 721, 796, 888, 889, 953). 

The three basic embryonic envelopes are as follows (Figs. 7.4 and 7.11)  

the shell material is described as being deposited against the inside of the capsule (204). The shell/capsule is well developed in the Pseudophyllidea, Tetraphyllidea and Trypanorhyncha. It is often poorly developed or absent in the Cyclophyllidea (Fig. 7.1(a)), in which case (especially in the Taeniidae) the embryophore (see below) is thickened and essentially functions as a protective shell . A subshell membrane has been described in some eggs (Fig. 7.1(a)). 

The outer envelope a complex layer filling the space between the capsule and the inner envelope (Fig. 7.11). 

The inner envelope a syncytial layer showing much variation. Some workers divide this layer into two zones - zone I, a cytoplasmic layer and zone II, a gelatinous layer (Fig. 7.14). Part of this embryonic layer gives rise to the embryophore (Fig. 7.4) and also to the oncospheral membrane (Figs. 7.4, 7.11 and 7.14) (a very thin layer surrounding the oncosphere), which is often counted as a fourth layer. Additional layers, which may be further derived from the above basic envelopes have been reported in some species (e.g. H. nana 204), but it is beyond the scope of this text to discuss all the various modifications which can occur. Only those features which have a special physiological significance are discussed below. 

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Substantial research efforts have been made to relate molecular properties to the structures of the junction zones and provide mechanisms for gel formation. Thus we have models for the formation and alignment of double helices for kappa-carrageenan, formation of triple helices for gelatin and egg box models for calcium induced gelation of alginate and pectin to mention a few examples (Morris 1986 Djabourov... 

In general, the remodeling of sperm chromatin that accompanies stage I decondensation involves the replacement of sperm-specific basic proteins with histones from the egg and results in the formation of nucleosomes. It now seems clear, at least in amphibian egg and Drosophila embryo extracts, that this remodeling is mediated by specific factors that participate in both assembly and disassembly processes. These changes in chromatin composition and structure can be analyzed by one- or two-dimensional polyacrylamide gel electrophoresis and micrococcal nuclease digestion. 

After plate formation, the structure of the PAM can be studied using the scanning electron micrograph (SEM) technique. Normally, Pb02 particles in the paste have three forms spherical, egg shaped, and crystal needle-like particles. Figure 3.3 shows the schemes for positive paste structures. 

Silk fibroin (SF) polymers consist of repetitive protein sequences and provide structural roles in cocoon formation, nest building, traps, web formation and egg protection. Silks are generally composted of P-sheet structures due to the dominance of hydrophobic domains consisting of short sidechain amino acids in the primary sequence. Silk is biocompatible, degradable and shows superior mechanical properties. Silk materials are amenable to aqueous or organic solvent processing and can be chemically modified to suit a wide range of biomedical applications [249-251]. 

The three globular proteins jS-lactoglobulin A, hen egg-white lysozyme, and bovine pancreatic ribonuclease A all have similar molecular weights. The whey protein jff-lactoglobulin has been the subject of numerous publications owing to the important role it plays in the fouling of metal surfaces and in the formation and stabilization of dairy foams and emulsions. " The structure, determined by X-ray crystallography, consists... 

Turner F.R. and Mahowald A.P. 1976. Scanning electron microscopy of Drosophila embryogenesis. 1. The structure of the egg envelopes and the formation of the cellular blastoderm. Dev. Biol. 50 95-108. Zhang C.X., Lee M.P., Chen A.D., Brown S.D., and Hsieh T. 1996. Isolation and characterization of a Drosophila gene essential for early embryonic development and formation of cortical cleavage furrows. /. Cell Biol. 134 923-934. 

It is possible to observe effects of organotin(lV) compounds exposure such as inhibition of cleavage of fertilized eggs, interference with the formation of the mitotic spindle, damages affecting chromosome structure, and electron-dense precipitate formation in organelles. 

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